Solid-electrolyte tantalum capacitors have been a major contributor to the miniaturization of electronic circuitry. They also operate over a wide temperature range and have good shelf life, long service and are useful in extreme environments.
Such capacitors are typically manufactured by compressing tantalum powder into a pellet and sintering the pellet to form a porous body. The porous body is then anodized in a suitable electrolyte to form a continuous dielectric oxide film on the sintered body. The pores are filled with an electrolyte and a lead wire is attached to form the capacitor.
In order to improve the specific capacitance (and volumetric efficiency) the particle sizes of the tantalum powder used to form the capacitors have been reduced to a smaller and smaller size. With the reduction in size, the temperature required to sinter the particles has dropped dramatically. However, it is also necessary to raise the temperature of the particles high enough during sintering to purify the particles, which has in turn limited the smallest particle size capable of being used so as to not excessively fuse it during the high temperature purification.
Another problem associated with using very small particles is encountered during the subsequent anodization of the capacitor anode body. Because the anodization process consumes the anode metal in an amount proportional to the anodization voltage, the requirements for specified voltage ratings implies severe limitations on how small the particles in a capacitor can be. Particles smaller than a critical size will be totally consumed by the anodization process.
Accordingly, there is a constant search in this art for particles with increased surface area which would be able to withstand temperatures necessary during sintering for cleaning and purification without excessive fusing and loss of surface area, and which will be stable during the anodization process.